Question

Select all the statements that are consistent with the Endosymbiont Theory Select all the statements that are consistent with the Endosymbiont Theory Mitochondria and chloroplasts are semiautonomous organelles, with many similarities to bacteria. Mitochondria resemble bacteria in size and shape and are surrounded by double membranes, the inner of which has bacterial-type lipids. Chloroplasts have circular DNA molecules. Mitochondria rRNA sequences, ribosome size, sensitivities to inhibitors of RNA and protein synthesis, and type of protein factors used in protein synthesis are all similar to bacteria.

Answer 1

• Mitochondria and chloroplasts are semiautonomous organelles, with many similarities to bacteria.
• Mitochondria resemble bacteria in size and shape and are surrounded by double membranes, the inner of which has bacterial-type lipids.
• Chloroplasts have circular DNA molecules.

Step-by-step explanation:

The endosymbiotic theory essentially states that some organules of the eukaryotic cells, such as mitochondria and chloroplasts, were once free-living bacteria. Probably, these organisms must have been phagocytized, but not digested by another cell. On the contrary, these bacteria were able to adapt to their host in such a way that the two cells established a dependent relationship with each other.

Due to its similarities to other free-living bacteria, it has been speculated that chloroplasts must derivate from cyanobacteria (they both absorb sunlight and store the energy in ATP, and produce organic molecules) and that mitochondria must derivate from rickettsias (they produce ATP in the same manner, by using the Krebs Cycle and Oxidative Phosphorylation).

From the phagocytosis moment, these two cells created a new bond between them of mutual benefit, becoming dependant on each other, up to the point that they would not be able to survive without the other one.

This theory is supported by a few characteristics of the chloroplasts and mitochondria that suggest that they once were a free cell. For example,

• Both organelles present their own genetic material. This DNI is independent of the DNA of the cells, is bi-catenary and circular, identical to the DNA of the bacteria, and very different from the one of the eukaryotic cells.

• These organelles divide by binary fission and not by mitosis, and are capable of synthesizing their own ribosomes and organelles.

• Both organelles present a double membrane, a characteristic that reinforces the idea of being phagocyted. The internal membrane looks identical to the membrane of bacterias, while the external membrane looks like the eukaryotic one.

In fact, in this internal membrane are placed the energy centers, exactly as it occurs in bacterias membrane.

• Finally, the sizes of the organelles are similar to the size of some procaryotes.